During the towing of certain measurement devices towed by the ships, sometimes called “fishes” but generally called herein “equipment”, as for example sonars, elements called “dead weight” or “V fin” are to be used. Such equipment and dead weights are towed by means of winches and towing cables, herein called trolling line, ensuring the mechanical link as well as, the most often, the transfer of data and/or energy between the ship and the equipment.
These dead weights are generally used for reasons of stability and immersion and the dead weight is interposed between the equipment and the ship. This dead weight has for main functions the decoupling of the ship movements with respect to the equipment and the equipment immersion control. These dead weights may use gravity or hydrodynamics or both at a time to play their role.
A method commonly used to handle the equipment and the dead weight consists in anchoring the equipment at the end of the trolling line, then anchoring the dead weight on the same line at a variable distance, chosen by the user.
The launch with immersion in water is made either through a crane or through a gantry crane, in the following manner:                immersing the equipment,        unwinding the trolling line up to the place where the dead weight must be anchored to the trolling line,        immersing the dead weight,        continuing the unwinding of the line up to the wanted distance of positioning of the equipment with respect to the towing ship.        
The equipment may then be used.
The exit from water of the trailed/towed elements for their retrieval is made in the same manner but in the opposite order.
It is also known from the document W02008/043823 a device for the automatic anchoring and disanchoring of a fish (sonar emitter) along a trolling line with automatic passive immobilization of the fish with respect to the line and the passive locking thereof to the line.
These launch and retrieval operations have difficulties and dangers requiring staff on an adapted working platform of the ship. The handled elements are heavy and the ship is on motion (pitch, roll . . . ). The cable must be able to pass in at least one pulley or in any device fulfilling this role. Most often, this pulley is fixed to a gantry crane, or even a crane, which allows at the time of launch, bringing the equipment above the water before releasing/unwinding the line by means of a winch, in order to place the equipment in water. From then on, once the equipment in water, it becomes difficult to retrieve the line to anchor the dead weight thereto. This is also true at the time of retrieval of trailed/towed elements. Hence risks for the staff and for the equipment that is sometimes fragile.